This is my new found LOVE. I seriously don't know what I did before I found Canva. It is one of the easiest services to edit your blog graphics and it's free. You can also check out my Canva tutorial video.
PicMonkey is another awesome graphic design tool. It is simple to use and you do not need any kind of graphic design experience to figure it out.
You can use this site to purchase stock photos for your blog. They offer a wide variety of photos to choose from. You can either signup for their monthly membership plan, or purchase individual photos.